1. Field of the Invention
The invention relates to a retardation arrangement for converting an input radiation beam, incident from an input side of the retardation arrangement, into an output radiation beam which has over its cross section a spatial distribution of polarization states which can be influenced by the retardation arrangement and differs from the spatial distribution of polarization states of the input radiation beam, and to a microlithography projection exposure machine having at least one such retardation arrangement.
2. Description of the Prior Art Used
In order to increase the imaging power of projection exposure machines for microlithography, it is frequently advantageous to set polarization states in a targeted fashion within the illumination system and/or in the case of the projection objective. For example, it is possible for a polarization state provided by the radiation of the primary light source to vary in an undesired way controllable with difficulty during passage through the projection exposure machine. Contributors to this can be, for example, intrinsic birefringence (IDB) or strain birefringence (SDB), caused by mechanical strains, in calcium fluoride (CaF2) and other materials which can be used for transparent optical components in the deep ultraviolet (DUV) region. Likewise, antireflection coatings and reflective coatings (mirror coatings) can unfavourably alter the polarization state of the radiation such that, for example, a linear polarization state at the input of the illumination system is converted into an undefined elliptic polarization state at the exit of the projection objective. Since the optically polarizing effect of the components is generally not the same overall, the output polarization state is also generally not constant over the cross section of the radiation beam. Generic retardation arrangements can contribute to a compensation of such effects.
The polarization state of the radiation used for image generation is frequently also influenced in a targeted fashion in order to improve the imaging quality. Use may be made for this purpose of generic retardation arrangements in the illumination system and/or in the projection objective.
DE 195 35 392 (corresponding to EP 0 764 858 B1) discloses a generic retardation arrangement which is provided for use in the illumination system of a projection exposure machine operating in the deep ultraviolet region, and which generates an output radiation beam which contains radiation polarized substantially in a radial direction over its entire cross section. The radial polarization is well suited for objectives with a typical image-side numerical aperture (NA) of approximately 0.5≦NA≦0.7 and photoresist without antireflection coating for the purpose of optimizing efficiency of coupling into the resist material. Tangential polarization, in which the local preferred polarization direction is substantially perpendicular to the radial direction of the beam, is frequently preferred for optimizing the two-beam interference in the case of very high numerical apertures, and can likewise be set by means of suitable retardation arrangements. An embodiment, effective in transmission, for converting linearly polarized input radiation into radially polarized output radiation has a multiplicity of hexagonal half-wave plates made from birefringent material which are arranged in a surface-filling fashion and whose main crystallographic axes are aligned perpendicular to the direction of incidence of the input radiation such that each half-wave plate deflects the direction of polarization of the locally incident radiation in the direction of a radius which intersects the half-wave plate and is directed onto the optical axis of the retardation arrangement.
DE 101 24 803 (corresponding to U.S. 2002/0176166 A1) discloses a retardation arrangement which is provided for comparable purposes, is effective in transmission, and consists in one embodiment of a transparent plate made from birefringent material on the entrance side and exit side of which there are in each case small regions with deflecting structures in the form of gratings or of refractive structures. The main crystallographic axis of the birefringent plate material is aligned parallel to the optical axis of the retardation arrangement, and thus substantially parallel to the irradiation direction of the input radiation beam. The deflecting structures produce an oblique passage of radiation through the plate material. Output radiation beams with a cylindrically symmetrical polarization distribution (tangential or radial) can be generated, for example, from incoming circularly or linearly polarized light by means of locally varying setting of suitable angles of inclination between the transit direction and the main crystallographic axis as well as suitable directions of inclination and plate thicknesses.
The as yet unpublished German Patent Application DE 103 24 468.9 from the applicant describes microlithography projection exposure machines in which transparent retardation elements which have form-birefringent grating structures whose arrangement varies locally over their useful cross section can be used for the purpose of setting a desired polarization state of the radiation.